Decrypt PGP string in C #

Question

I searched for an answer four hours later and cannot find anything.

This will usually be easy, but:

We cannot write files or call an external file.

We cannot use the BouncyCastle API.

Most of the libraries I need require a call to pgp.exe (or similar), which is out of the question.

We have public / private keys and encrypted strings are transmitted.

Is there an easy way to decrypt this in C # /. NET4?